Protection switching is a network survivability technique with dedicated protection resource allocation. Examples of this for ATM may be read in ITU-T Recommendation I.630 “Integrated services digital network (ISDN) maintenance principles. ATM protection switching”, February 1999. Protection switching according to I.630 may be implemented also in other systems in a similar way.
In ITU-T Recommendation G.841 “Digital networks. Types and characteristics of SDH network protection architectures”, July 1995, more examples of protection switching can be found.
A protected domain defines one or more units, for which a survivability mechanism is provided in the event of an impairment affecting that or those units. The concept “unit” should in this context be interpreted broadly and may for example be a switch port, an external switch or router link, groups of data flows or an individual data flow. The protected domain normally begins after a selector, bridge or similar of one endpoint—a source—and extends to a selector, bridge or similar of another endpoint—a sink. A protected domain may thus extend e.g., across a network connection, across a subnetwork connection or across a single link connection.
A working entity is the portion of a unit that is within the protected domain from which working traffic is received at the sink of the protected domain under fault-free condition.
A protection entity is the portion of a unit that is within the protected domain from which working traffic is received at the sink of the protected domain where a working entity has failed.
A protection switching architecture may be of e.g., a 1+1 type or an m:n type.
In the 1+1 architecture type, “hot” standby, a protection entity is dedicated to the working entity with the working traffic bridged onto the protection entity at the source of the protected domain. The traffic on working and protection entities is transmitted simultaneously to the sink of the protected domain, where a selection between the working and protection entity is made based on some predetermined criteria, such as a server defect indication. An example of 1+1 may be found in EP 1 056 309 A1.
In the m:n architecture type, “cold” standby, m dedicated protection entities are shared by n working entities, where m≦n typically. The bandwidth of each protection entity should be allocated in such a way that it may be possible to protect any of the n working entities in case at least one of the m protection entities is available. When a working entity is determined to be impaired, it first must be assigned to an available protection entity followed by transition from the working to protection entity at both the source and sink of the protected domain.